1. Field of the Invention
The present invention relates to optical scanners and more particularly to means for illuminating a scanned transverse region of a moving web or the like.
2. Description Relative to the Prior Art
It is well known in the prior art to detect the presence of defects, splices or other irregularities or discontinuities in a moving web by illuminating a transverse region of the web and scanning a region within the illuminated region. Such scanning may be performed by an electronic camera employing either a linear array of photodetectors that view corresponding areas of the illuminated region and that are read out serially, or by means of a single photodetector combined with an optical system that sweeps the viewing area of that detector along the illuminated region.
To detect relatively small or subtle irregularities in a rapidly moving web, the illumination must be of relatively high intensity. For example, U.S. Pat. No. 4,752,897, issued on June 21, 1988 in the names of Leon Zoeller et al. discloses such an apparatus capable of scanning a web at extremely high speeds, requiring handling of the resulting digital signals at data rates of the order of tens of megabytes per second. This rquires quite brilliant illumination because of the extremely short duration during which each individual area of the web or the like is accessed. Correlatively, the illumination must be uniform across the width of the web so that variations in the intensity of the light received by the photodetector are attributable to variations in the web and not to variations in its illumination.
One previously known way to provide the required intense illumination of the web is by means of a laser light source that sweeps a beam of light across the web in synchronism with the detector means. Such a device, however, is inherently expensive. Also, this approach may be limited in its applications because of the coherent monochromatic nature of the light itself, which makes it unsuitable for detecting particular types of defects.
Another known way to illuminate the web is by means of an elongate fluorescent lamp tube located close to the web. Because the entire transverse scanning region of the web is constantly illuminated, as opposed to only a small illuminated area swept along that region, more time is available for each detector element of a linear array type detector to monitor the corresponding scanning area and the light intensity can be correspondingly lower. However, even this lower intensity requirement may not be achievable with a fluorescent tube because such a tube has no localized high intensity light source that can be focused onto the web. In other words, such a tube provides a large area source of relatively low intensity illumination which cannot be intensified by optical means.
U.S. Pat. No. 3,739,219, issued on June 12, 1973, discloses an elongate incandescent light tube intended to replace a plurality of conventional incandescent lamp bulbs to produce an elongate zone of illumination of substantially uniform intensity through which multiple channel punched cards or punched tape pass. One or more elongate filaments are tensioned within the transparent lamp tube by conductive leaf spring members extending into tubular legs extending laterally from the tube at opposite ends thereof. There is no suggestion in this disclosure that the illumination need be of high intensity and it is obvious from FIG. 6 of that patent that this is not a requirement in view of the relatively long distance between the lamp and the photodetector and the absence of any means for focusing light from the lamp filament onto the photodetectors.
U.S. Pat. No. 1,041,100, issued on Oct. 15, 1912, discloses an elongate incandescent lamp with one or more filaments stretched between conductive coil tension springs. The lamp is indicated as being of the type "adapted to be employed with a cylindrical reflector for certain decorative or other purposes of illumination," but there is no suggestion of means for imaging the filament at a remote location or for using the lamp in a web scanning device or the like. U.S. Pat. No. 1,104,807, issued on July 28, 1914, discloses a similar lamp, with no specific indication of its intended use and no reference to reflectors or other light directing means. In this disclosure, one or more filaments are likewise stretched between two conductive tension coil springs, but shunts are placed around the coil springs so that the latter are not required to conduct all of the filament current. In both of these references, at least one end of the filament or multiple filament assemblage is supported by the corresponding tension spring, which causes any fabrication inaccuracy or thermal distortion of the spring to affect the critical filament location.